Communication systems utilizing multiple communication structures are useful in providing multiple telecommunication links across widely dispersed locations. To exploit this benefit, the multiple links must be separated from each other in time and/or frequency. Various methods of multiple access are available to implement the separation of telecommunication links.
In frequency division multiplexing/time division multiplexing methods of signal access, a communication system may be structured to have subbands and time slots. The frequency spectrum is divided into frequency channels where each frequency channel occupies a certain frequency range, and a certain number of frequency channels are combined into a subband. Based on the available frequency subbands and number of time slots implemented in the system, and constraints such as the interference level permitted, the availability of communication channels for assignment to users is limited. As defined herein, the term "reuse unit" is a time slot and subband combination providing an available communication channel to a user.
"Time shift" (or "time slide") is a phenomenon that is created by the propagation path delay of transmission from satellites to other communication structures. Typically, the greater the distance between the satellite and the other communication structure, the greater the time of propagation path delay. When a user is communicating in a satellite communication system, the user may be located in coverage areas of a multiple number of satellites. For example, the user may receive a desired transmission signal in an allotted, receive time slot and additionally receive other signals transmitted from the satellites having the user in their coverage areas in the same allotted, receive time slot. These other signals received by the user are interfering signals and are said to have their signal time shift into the user's allotted receive time slot. The extent of time shift depends on propagation paths between the user and all the other satellites having the user in their coverage areas and depends on the design procedures of assigning time slots in a channel reuse unit allocation.
Another possibility for the occurrence of time shift is due to the generation of interference signals from the assignment of the same frequency channels and time slot location to other users and consequently the realization of frequency subband reuse technology and/or due to system nonlinear operations (e.g., antenna power amplification and frequency conversion) when the signal is transmitting through the system.
Furthermore, in a satellite communication system, for example, the position of the satellites complicates the problem of time shift. For example, as satellites converge, transmission overlap increases and the amount of time required to separate the transmissions may increase. As adjacent-plane satellites diverge, transmission overlap decreases and the amount of time required to separate the transmissions may decrease.
Prior-art methods do not adequately address the problem of time shift. They simply do not allocate reuse units adjacent to the assigned reuse unit for signal transmission in order to compensate for the time shift. The time slots of the reuse units adjacent to the assigned reuse unit are inefficiently used. Typically, the prior-art methods increase the delay time of signal transmissions to account for time shift. In addition, the prior-art methods do not address the problem of varying degrees of time shift.
When the system traffic is heavy, a reuse unit may endure different levels of interference within the duration of the reuse unit because of the realization of frequency subband reuse technology and/or due to system nonlinear operations. As a result, the user encounters different levels of communication quality, for example. The system performance is not guaranteed uniformly in the full reuse unit time duration. The prior-art methods do not address the problem of providing uniform communication quality.
What are needed are a method and apparatus for signal transmission that maximize communication channel assignment to efficiently minimize the degree of unused time duration of signal transmissions. Further needed are a method and apparatus for signal transmission that flexibly and dynamically accommodate various degrees of potentially overlapping transmission due to varying satellite locations. Further needed are a method and apparatus to provide uniform signal transmission during a user's access to the communication system.